Built 273, what sure grip ratio with a A833 3.09 first gear

I've got an Aussie Hemi 265 6 cylinder that I'm building and will be turbo'd. I bought a 1969 b-body A833 and it's got the 3.09 gears.
Why is 3.09 good for 6 cylinders? and will a 3.54 rear be ok?